The present invention relates to an electrostatic capacitance type dynamic quantity sensor for detecting a dynamic quantity such as pressure, acceleration, etc.
As an example of conventional pressure sensors, there is an electrostatic capacitance type pressure sensor of surface device processing type as disclosed in JP B 7-50789, for instance. This pressure sensor is formed by forming a first electrode (fixed electrode) by diffusing impurities on a monocrystal silicon substrate, and arranging a second electrode (movable electrode) of diaphragm shape formed of poycrystal silicon having conductivity on the monocrystal silicon substrate so as to oppose the first electrode with an air gap therebetween, whereby pressure induced displacement of the second electrode changes the electrostatic capacitance, thereby allowing detection of the pressure.
FIG. 7 shows a section of a pressure detecting portion of the above-mentioned electrostatic capacitance type pressure sensor.
As shown in FIG. 7, a fixed electrode 122 is formed as a diffusion layer on a monocrystal silicon substrate 121, and a movable electrode (diaphragm) 131 is arranged above a protective film 123 and an air gap 125. The movable electrode 131 is composed of protective films 126, 129 of the nitrified film or the like and a conductive layer 128 of polycrystal silicon.
Signals (electric signals) of electrostatic capacitance between the movable electrode 131 and the fixed electrode 122 are taken out through aluminum wiring or the like. Assuming that an opposing surface area between the movable electrode 131 and the fixed electrode 122 is S and a distance of air gap between the fixed and movable electrodes 122, 131 is d, an electrostatic capacitance value Cs of this capacitance conversion element is expressed by the following equation (1)
Cs=∈0xc2x7S/dxe2x80x83xe2x80x83(1)
where ∈0 is dielectric constant in vacuum.
When pressure is applied onto the movable electrode 131, the air gap d between the fixed electrode 122 and the movable electrode 131 changes and the sensor capacitance Cs changes. However, in the case where the capacitance Csis converted into an electric signal without modification of the above equation (1), the larger the sensitivity is made (the larger the distance d is changed) the larger is the degree of non-linearity of characteristics of change in the capacitance Cs to the pressure inputted. Therefore, even if one tries to make a larger change in electrostatic capacitance to input pressure in order to achieve both of the electrostatic capacitance sensor small in size and improving a S/N ratio, the above-mentioned non-linear characteristic becomes a bar to realizing it.
Further, since the electrode is formed on the silicon substrate (in this case, it is formed by diffusion), the floating capacitance between the fixed electrode 122 and the silicon substrate 121 and between the movable electrode 131 or the wiring 130 and the silicon substrate 121 becomes large relative to the sensor capacitance Cs.
As a method of solving the non-linear characteristic problem of the electric signal, a system of obtaining an output proportional to a reciprocal of the electrostatic capacitance Cs has been already proposed, for example, it is a circuit disclosed in Sensors and Actuators A 60 (1997) page 32 to 36. This is constructed so that a circuit for converting electrostatic capacitance changing by pressure is constructed by integration feedback capacitance of an operation amplifier, whereby an electric charge charged onto the integration feedback capacitance is converted into a pressure signal.
FIG. 8 shows the conventional circuit. In FIG. 8, a reference (standard) number 1 denotes a constant voltage source, 2a and 3a each denote a changeover switch, 4 denotes a reference electrostatic capacitance element the capacitance CR of which is fixed, 5 denotes a dynamical quantity detecting electrostatic capacitance element which is formed by a movable electrode and a fixed electrode and in which the electrostatic capacitance, that is, Cs changes according to a dynamic quantity and 7 denotes an operational amplifier.
The reference electrostatic capacitance element 4 is connected to a reverse input terminal of the operational amplifier 7, and the dynamic quantity detecting electrostatic capacitance element 5 (electrostatic capacitance Cs) is provided in a feedback circuit between the reverse input terminal and an output terminal of the operational amplifier. The switches 2a, 3a are elements of a charge discharge circuit of the electrostatic capacitance CR, Cs, and positioned at places of solid lines (in FIG. 8 ) when timing is xcfx861 and positioned at places of broken lines (in FIG. 8) when timing is xcfx861B.
According to this circuit, when timing is xcfx861, a voltage value Vcc of the constant voltage source 1 is applied to the reference electrostatic capacitance element 4 through the switch 2a, and charged charges are integrated in the dynamic quantity detecting electrostatic capacitance element 5. When timing is xcfx861B, the charges charged in the reference electrostatic capacitance element 4 are discharged through the switch 2a and the charges charged in the dynamic quantity detecting electrostatic capacitance are discharged through the switch 3a. By repeating the above-mentioned two modes, pulse like output signals are obtained at the output terminal 9.
Output Vout of the circuit is expressed by the following equation (2)
Vout=(CR/Cs)Vcc=xe2x88x92(SRds/SSdR)Vccxe2x80x83xe2x80x83(2)
where SS denotes an area of the dynamic quantity detecting electrostatic capacitance element, ds denotes a distance (air gap) between the electrodes of the dynamic quantity detecting electrostatic capacitance element, SR denotes an area of the reference electrostatic capacitance element and dR denotes a distance (air gap) between the electrodes of the reference electrostatic capacitance element.
Accordingly, since it is constructed so that output voltage changes proportionally to a reciprocal of capacitance Cs of the dynamic quantity detecting electrostatic capacitance element, that is, proportionally to a change of an air gap ds, the output becomes an excellent characteristic without non-linearity in principal. Such a circuit is disclosed in JP A 4-329371 and JP A 5-18990, for example.
In this case, electrostatic capacitance Cs for dynamic quantity detection has turned to be integration feedback capacitance of the operational amplifier, so that driving frequency for detection is restricted according to response speed of the operational amplifier. In order to precisely convert very small capacitance (1 pF or less) into an electric signal, it is necessary to detect dynamic quantity by high speed detection driving frequency (several hundreds kHz or higher). However, in the event that the detection driving frequency is restricted according to response speed of the operational amplifier, as mentioned above, a high speed operational amplifier is needed to detect capacitance at high frequency and with high precision, which is increases the cost and makes large in size.
Further, where an element of large floating capacitance (as shown in the first prior art) is driven, since the floating capacitance becomes a bar to improving on the response of the operational amplifier and stability, finally, the conventional sensor is not suitable for detection of very small capacitance with high precision. Further, in order to obtain D.C. output, it is necessary to add a sample and hold circuit to a rear stage.
Further, JP A 6-507723 (Laid-open PCT application) discloses a pressure measurement apparatus in which a ratio of a difference between sensor capacitance Cs and reference capacitance CR is taken by reference capacitance Cf divided of an electrode displaced by pressure. Transmission function F deriving pressure by capacitance measurement in this case is expressed by the following equation (3):
F=(Csxe2x88x92CR)/Cfxe2x80x83xe2x80x83(3)
This prior art intends to compensate an error portion not completed to correct by 1/C function, or non-linearity by dividing an electrode of electrostatic capacitance changing by pressure (in other words, division of the capacitance into Cs and Cf). In this case, also, since feedback integration capacitance Cf influences an output of dynamic quantity detection, a high speed response operational amplifier is needed to detect very small capacitance at high frequency and with high precision, and a sample and hold circuit, etc. are needed at a rear stage of the capacitance detection circuit to take out the output as direct current.
The present invention is made by noticing the above-mentioned various problems, and an object of the present invention is basically to realize an electrostatic capacitance type dynamic quantity sensor which is able to detect very small electrostatic capacitance (dynamic quantitative displacement) by using high speed detection driving frequency without restriction of response of an operational amplifier, stably and at high speed without influence of floating capacitance and by converting it straightly into voltage. Further, another object of the invention is to provide an electrostatic capacitance type dynamic quantity sensor which is able to obtain D.C. output without adding a sample and hold circuit at a rear stage.
The present invention is constructed basically as follows. As for symbols added to elements constructing according to the present invention, symbols used for an embodiment of FIG. 1 are used here for convenience.
That is, a sensor according to the present invention comprises a dynamic quantity detecting electrostatic capacitance element 5, of which the electrostatic capacitance Cs changes according to a dynamic quantity, a reference electrostatic capacitance element 4 forming reference electrostatic capacitance CR and a capacitance detection circuit (it corresponds to an integration feedback capacitance CF and an operational amplifier 7 in FIG. 1) outputting a dynamic quantity signal proportional to a reciprocal of electrostatic capacitance Cs of the dynamic quantity detecting electrostatic capacitance element 5. The sensor is characterized in that the circuit is constructed so that when the dynamic quantity detecting electrostatic capacitance element 5 displaced according to a dynamic quantity, electric charge quantities charged between the dynamic quantity detecting electrostatic capacitance element 5 and the reference electrostatic capacitance 4 become unbalanced to produce a difference therebetween, the output Vout of the capacitance detecting circuit changes according to the difference in electric charge, and a charge and discharge circuit (which corresponds to a constant voltage power source 1, an adder 8, and changeover switches 2, 3 in FIG. 1) is operated so that, using the output Vout of the capacitance detecting circuit, the output changes until electric charge quantities of the dynamic quantity detecting electrostatic capacitance element 5 and the reference electrostatic capacitance 4 become equal to each other.
According to the above construction, when the dynamic quantity detecting electrostatic capacitance element 5 displaces according to a dynamic quantity, electric charge quantities charged between the dynamic quantity detecting electrostatic capacitance element 5 and the reference electrostatic capacitance 4 become unbalanced to produce a difference therebetween, and the output Vout of the capacitance detecting circuit changes according to the difference in electric charge, however, to conclude (details will be explained in the description of embodiments of the invention), finally, the output Vout becomes stable at a value at which electric charge quantities of the dynamic quantity detecting electrostatic capacitance element 5 and the reference electrostatic capacitance 4 are equal to each other. The output Vout is proportional to a reciprocal of electrostatic capacitance Cs for dynamic quantity detection and it is D.C. voltage. Further, in the case where the output Vout becomes stable at a value at which electric charge quantities of the dynamic quantity detecting electrostatic capacitance element 5 and the reference electrostatic capacitance 4 are equal to each other, even if an operational amplifier 7 with integration feedback capacitance (condenser for feedback) CF, output independent from the integration feedback capacitance value is obtained.